Conventionally, Ziegler-Natta catalyst systems comprising titanium or vanadium compounds as main catalyst and alkylaluminum compounds as cocatalyst have been used to prepare ethylene homopolymers or copolymers of ethylene and α-olefins. Although the Ziegler-Natta catalyst system provides good efficiency of ethylene polymerization, the resulting polymers tend to have a broad molecular weight distribution because of the heterogeneousness of catalytic active sites. In particular, the resulting copolymer of ethylene and an α-olefin has no uniform compositional distribution.
Recently, the so-called metallocene catalyst system, which comprises metallocene compounds of group IV transition metals, e.g. titanium, zirconium or hafnium, and methylaluminoxane as cocatalyst, has been developed. Because this catalyst system is a homogeneous cocatalyst having homogeneous catalytic active sites, it is capable of preparing polyethylenes having a narrower molecular weight distribution and a more uniform compositional distribution as compared to the Ziegler-Natta catalyst system. For example, European Patent Publication Nos. 320,762 and 372,632 or Japanese Patent Laid-Open Nos. Sho 63-092621, Hei 02-84405 and Hei 03-2347 disclose that polyethylenes having a molecular weight distribution (Mw/Mn) of 1.5-2.0 can be prepared efficiently from ethylene by activating the metallocene compounds Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene (IndH4)2ZrCl2, etc. with the cocatalyst methylaluminoxane. However, it is difficult to obtain high molecular weight polymers with this catalyst system. Especially, when applied to solution polymerization performed at high temperature, i.e. 140° C. or higher, it is not suitable to prepare high molecular weight polymers having a weight average molecular weight (Mw) of 100,000 or higher because the polymerization efficiency decreases rapidly and the elimination of β-hydrogen prevails.
Recently, the so-called geometrically constrained non-metallocene catalyst system (also known as the single-site catalyst) enables the preparation of high molecular weight polymers through homopolymerization of ethylene or copolymerization of ethylene and an α-olefin under the solution polymerization condition, in which a transition metal is connected as a ring. European Patent Publication Nos. 0416815 and 0420436 disclose a compound in which an amide group is connected to a cyclopentadiene ligand to form a ring.
And, European Patent Publication No. 0842939 discloses an electron donor compound catalyst in which a phenol-based ligand and a cyclopentadiene ligand are connected to form a ring. However, the geometrically constrained catalyst is inappropriate for commercial application because the yield of the ring forming reaction between the ligand and the transition metal compound in the synthesis of the catalyst is very low.
U.S. Pat. No. 6,329,478 and Korean Patent Publication No. 2001-0074722 disclose non-geometrically constrained, non-metallocene catalysts that can be used under hot solution condition. These patents disclose that single-site catalysts having at least one phosphinimine compound as ligand provide superior ethylene transition ratio during the copolymerization of ethylene and an α-olefin by solution polymerization under high temperature condition of 140° C. or higher. However, specific phosphine compounds have to be used to synthesize the phosphinimine ligand. These compounds are inappropriate for large-scale olefin polymer production because they are harmful to the environment and humans. U.S. Pat. No. 5,079,205 discloses a catalyst having a bis-phenyl oxide ligand, but it has too low a catalytic activity to be commercially applicable.
In addition, synthesis of a phenyl oxide ligand in which the phenyls of 2- and 6-positions are substituted by alkyl groups as non-metallocene catalyst and polymerization using the same are reported by Nomura et al. [Organometallics 1998, 17, 2152]. But, its application is restricted to the preparation of higher-grade copolymers of α-olefins in good yield, because of the steric hindrance by the substituents of the phenyl oxide ligand.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the related art that is already known in this country to a person of ordinary skill in the art.